Problem: Heather is a cashier. She can ring up $12$ customers in $9$ minutes. At this rate, how many minutes does it take her to ring up $4$ customers?
We're trying to find the number of minutes it takes Heather to ring up $4$ customers. $1$ $\longrightarrow$ $\dfrac34$ ${12}$ $\longrightarrow$ ${9}$ $ \div 12$ $ \div 12$ ${customers}$ $\longrightarrow$ ${minutes}$ If Heather can ring up ${12}$ customers in ${9}$ minutes, then she can ring up $1$ customer in $\dfrac34$ of a minute. $4}$ $\longrightarrow$ ${3}$ $1$ $\longrightarrow$ $\dfrac34$ $ \times 4$ $ \times 4$ ${12}$ $\longrightarrow$ ${9}$ ${customers}$ $\longrightarrow$ ${minutes}$ So, Heather can ring up $4}$ customers in ${3}$ minutes.